1. Field of the Invention
The present invention relates to optical transmission equipment, and more particularly, to optical transmission equipment that prevents malfunction derived from communication of fault information between transceivers.
2. Description of Related Art
In the past, optical transmission systems have been designed on the assumption that audio signals are transmitted over a trunk line including multiple telephone lines, and requested to provide super-reliable, very long-distance, and high-definition performance. On the other hand, there is a demand for low-cost data transmission targeted on base-to-base communication in a firm or interconnection between local area networks (LANs). An optical transmission system designed for the low-cost data transmission has been demanded and actively introduced. The data transmission techniques are based on the Ethernet technology. The specifications for optical transceivers concerning the products and characteristics of the optical transceivers are made public so that products of a plurality of optical transceiver manufacturers will be compatible with one another. Moreover, when a plurality of vendors provide devices, modules, and pieces of equipment, a low-cost system can be realized. Some optical transceiver manufacturers apply unique specifications to their optical transceivers.
The related art of the present invention will be described in conjunction with FIG. 1 to FIG. 5. FIG. 1 is a block diagram explanatory of the configuration of a conventional bidirectional optical transmission system. FIG. 2 is an explanatory diagram concerning actions to be performed in the conventional bidirectional optical transmission system in case a fault takes place. FIG. 3 is a state transition diagram explanatory of a fault notification facility to be included in a conventional optical transceiver. FIG. 4 is a sequence diagram explanatory of the fault notification facility of the conventional optical transceiver. FIG. 5 is a state transition diagram explanatory of another fault notification facility to be included in the conventional optical transceiver.
FIG. 1 shows the configuration of a bidirectional optical transmission system employing two-conductor optical fibers. An optical transceiver 110 comprises an optical transmitter 111 and an optical receiver 112, and an optical transceiver 120 comprises an optical transmitter 121 and an optical receiver 122. The optical transceiver 110 and optical transceiver 120 are linked by two-conductor optical fibers 131 and 132. Thus, optical transmission between two points is realized.
Now, a case where a fault takes place on one of the communication links included in the bidirectional optical transmission system shown in FIG. 1 will be discussed below. If the optical fiber 131 is broken or if the optical fiber 131 is incorrectly coupled to the optical receiver 122, the optical receiver 122 cannot receive any optical signal. However, since the communication link of the optical fiber 132 is held intact, no problem occurs in reception by the optical receiver 112. Therefore, although the fault has occurred, the optical transceiver 110 is unaware of the occurrence of the fault.
In general, optical transceivers are designed so that if a fault occurs, a special signal will be transmitted in addition to data that should be conveyed. Referring to FIG. 2, transfer of signals in case of a fault will be described. In FIG. 2(a), for example, if a fault occurs on the optical fiber 131, the optical receiver 122 detects the fault. In FIG. 2(b), the optical transmitter 121 initiates transmission of a first fault notification signal (hereinafter called a fault detection signal), which signifies that a fault has been detected, to the optical receiver 112. When the optical receiver 112 detects the fault detection signal, the optical transceiver 110 recognizes occurrence of the fault. Furthermore, the optical receiver 120 having detected interception of a signal recognizes that both the remote transmitter 111 and local receiver 122 have detected the fault. Moreover, the optical receiver 110 having detected the fault detection signal recognizes that both the local transmitter 111 and remote receiver 122 have detected the fault. Thus, the optical transceivers 110 and 120 can locate a faulty component.
Moreover, the optical transceiver 110 having received the fault detection signal must suspend data transfer because the fault has occurred downstream the local optical transmitter 111. On the other hand, the optical transmitter 111 must continuously transmit a certain signal so that immediately after the faulty component linking the optical transmitter 111 and optical receiver 122 is recovered to enable communication, the fact that the faulty component is recovered can be recognized. Therefore, a signal other than the fault detection signal, which signifies that data transfer is suspended because the fault detection signal has been detected and a standby state is under way (hereinafter called a standby signal), is adopted as a second fault notification signal. This method is widely adopted. After the optical receiver 112 detects the fault detection signal as shown in FIG. 2(b), the optical transmitter 111 suspends data transfer and transmits the standby signal instead.
FIG. 2(C) shows a state established immediately after the faulty component is recovered. Since the faulty component is recovered, the optical receiver 122 detects the standby signal. When the standby signal is detected, the optical transmitter 121 resumes data transmission. In FIG. 2(d), when the optical receiver 112 receives data instead of the fault detection signal, the optical transmitter 111 resumes data transmission.
As mentioned above, the optical transceiver 110 and optical transceiver 120 that are opposed to each other check occurrence of a fault and locate a faulty component. When recognizing that the faulty component has recovered, the optical transceivers resume bidirectional data communication.
When the foregoing change in the state of an optical transceiver is summarized, it is plotted like the state transition diagram of FIG. 3. The normal state is state 0 in which data is transmitted. In this state, if an optical transceiver detects a fault, the optical transceiver changes the state thereof into state 1 and transmits the fault detection signal. In state 1 or state 0, if the optical transceiver receives the fault detection signal, it changes the state thereof into state 2 and transmits the standby signal. In state 1 or state 0, if the optical transceiver receives data or the standby signal, it returns to state 0 and resumes data transmission.
Referring to the state transmission diagram, a procedure to be followed by optical transceivers in case a fault takes place and a procedure to be followed thereby after a faulty component is recovered will be described in conjunction with the sequence diagram of FIG. 4. In FIG. 4(a), if a fault occurs, the optical transceiver 120 detects the fault, changes states from state 0 to state 1, and transmits the fault detection signal. Thereafter, the optical transceiver 110 detects the fault detection signal, changes states from state 0 to state 2, and transmits the standby signal.
In FIG. 4(b), after the faulty component is recovered, the optical transceiver 120 detects the standby signal. The optical transceiver 120 then changes states from state 1 to state 0 and resumes data transmission. The optical transceiver 110 then detects data, changes states from state 2 to state 0, and resumes data transmission.
An example of a facility for detecting a fault and recovering a faulty component, there is, for example, a fault notification facility to be adapted to the Ethernet having a throughput of ten gigabits per second. The Institute of Electrical and Electronic Engineers of the U.S. has stipulated as a standard IEEE802.3ae the specifications for the fault notification facility for the 10 Gbps Ethernet. This document reads “detection of a local fault” in place of “DETECTION OF FAULT” described in FIG. 3, reads “transmission of a remote fault signal” in place of “TRANSMISSION OF FAULT DETECTION SIGNAL” described in FIG. 3, reads “reception of the remote fault signal” in place of “RECEPTION OF FAULT DETECTION SIGNAL” described in FIG. 3, reads “transmission of an idle signal” in place of “TRANSMISSION OF STANDBY SIGNAL” described in FIG. 3, reads “reception of data or the idle signal” in place of “RECEPTION OF DATA OR STANDBY SIGNAL” described in FIG. 3, and describes that a faulty component is located and recovered according to the same mechanism.
The fault detection facility that uses two signals of the fault detection signal and standby signal has been described so far. Improvement of safety using the fault detection facility has been discussed in many aspects. Referring to FIG. 2(b), the optical transmitter 111 continues transmission of a standby signal until a faulty component is recovered. Conceivable as the cause of the fault is the failure of the optical transmitter 111 or optical receiver 122, of the breakage or incorrect coupling of the optical fiber 131. Except the case where the optical transmitter 111 has failed, the standby signal may be released as an optical signal to a space outside equipment during a period during which a fault takes place or work of recovering a faulty component is in progress. As a means for minimizing the adverse effect of the release of the optical signal to the space outside equipment, a technique of suppressing the optical power of the standby signal has been proposed.
For example, a document, “Evaluating Open Fiber Control” (Ken Herrity, [online], September, 2000, IEEE802.3ae 10 Gb/s Task Force Plenary Meeting, [retrieved on June, describes a technique for the 10 Gbps Ethernet for suppressing a means optical power by intermittently transmitting a standby signal. FIG. 5 is a state transition diagram concerning the technique. When a fault detection signal is received, an optical transceiver changes the state thereof into state 2. The standby signal is then transmitted. If data or the standby signal is not received for a certain period of time (T1), a faulty component is recognized not to have been recovered. The optical transceiver then changes the state thereof into state 3. In state 3, transmission of the standby signal is suspended because there is a possibility that light is released to the space outside equipment over a downstream optical fiber (optical output is intercepted). However, as long as state 3 persists, when the faulty component is recovered, an opposite transceiver cannot receive the standby signal. Consequently, communication cannot be resumed. Therefore, the optical transceiver returns to state 2 again after elapse of a certain period of time (T2) and transmits the standby signal. As long as the faulty component is not recovered, the state of the optical transceiver continuously changes between state 2 and state 3. Optical powers are evened between an on period (T1) during which light is propagated and an off period (T2) during which light is intercepted. For example, if the T1 and T2 values are equal to each other, a mean optical power is a half of an original optical power. If the T2 value is nine times larger than the T1 value, the mean optical power is suppressed to be a one-tenth of the original optical power.
In FIG. 5, even if a data signal or the standby signal is received in state 3, state 3 is not changed to state 0. This is because after the standby signal is transmitted in state 2, since no response is returned within the certain period of time (T1), negotiation or handshaking is thought to be reset at the same time when a transition is made to state 3.
Moreover, Japanese Unexamined Patent Application Publication No. 2001-217778 describes a method adopting as a standby signal a signal whose duty factory is small (short pulse train) and a technique for suppressing the power of the standby signal itself by employing a signal whose level or power itself is low. This method or technique refers to a case where a special signal whose power itself is different from that of a data signal or a fault detection signal is adopted as the standby signal to be transmitted in state 2 shown in the state transition diagram of FIG. 3.
Japanese Unexamined Patent Application Publication No. 05-206945 describes an optical transceiver effective in extending the service life of a light-emitting device by disabling the light-emitting device from working when the absence of a main signal in two directions is found by monitoring the level of a received signal.
Japanese Unexamined Patent Application Publication No. 2004-015084 describes wavelength-division multiplexing transmission equipment that prevents a deadlock from occurring between transponders.
Japanese Unexamined Patent Application Publication No. 2003-110585 describes an Ethernet terminal that detects occurrence of a fault on a transmission line between terminals interconnected over the Ethernet and that even when disconnecting a link with an opposite terminal, does not notify the opposite terminal of the fact.
Japanese Unexamined Patent Application Publication No. 2002-057635 describes optical signal monitoring equipment that when receiving a fault notification signal contained in an optical signal sent from upstream equipment, intercepts optical output to associated downstream equipment.
Problems the present invention attempts to solve will be described in conjunction with FIG. 6 to FIG. 8. FIG. 6 is a block diagram of a wavelength-division multiplexing system having optical transceivers and pieces of wavelength-division multiplexing transmission equipment interconnected. FIG. 7 and FIG. 8 are sequence diagrams explanatory of a fault notification facility of each optical transceiver.
In order to realize transmission of a larger throughput using an Ethernet optical transceiver, the use of the optical transceiver in combination with wavelength-division multiplexing (WDM) transmission equipment would prove effective. The WDM is a method of combining a plurality of optical signals having different wavelengths, and transmitting the optical signals over a single optical fiber. In the WDM, as the number of wavelengths to be multiplexed gets larger, a total transmission throughput increases proportionally. This permits an optical fiber to exhibit a large data-carrying capacity.
When wavelength-division multiplexing transmission equipment and an optical transceiver are interconnected, the wavelengths of light to be transmitted by the optical transceiver are limited as described below. First, the bandwidth of light to be transmitted by the wavelength-division multiplexing transmission equipment is limited depending on the bandwidth of light to be transmitted over an optical fiber or the bandwidth of light to be amplified by an optical amplifier for long-distance transmission. Moreover, when the number of wavelengths to be multiplexed is increased, the difference between adjacent wavelengths gets smaller. This brings about a crosstalk between signals. Therefore, the wavelength of each signal must be strictly managed in the order of nanometers. As for the wavelength of each signal, any of specific wavelengths set in the form of, generally a “grid” is adopted. On the other hand, the wavelengths of signals to be transmitted by an optical transceiver that does not support wavelength-division multiplexing, such as, a general Ethernet optical transceiver are defined in the specifications for the optical transceiver to encompass an error of several tens of nanometers or more. Consequently, when the Ethernet transceiver is directly connected to the wavelength-division multiplexing transmission equipment, the crosstalk is intensified and the band use efficiency is deteriorated. At the worst, even reception may be hard to do.
When an optical transceiver that does not support wavelength-division multiplexing transmission (that does not manage wavelengths in the order of nanometers) must be connected to wavelength-division multiplexing transmission equipment, a device called a transponder is connected between the optical transceiver and wavelength-division multiplexing transmission equipment in order to realize a configuration like the one shown in FIG. 6. Wavelength-division multiplexing transmission equipment 141 comprises a multiplexer 142 that multiplexes a plurality of wavelengths and a demultiplexer 143 that separates a signal, which has wavelengths multiplexed, into signals of different wavelengths. The wavelength-division multiplexing transmission equipment 141 is opposed to wavelength-division multiplexing transmission equipment 151, which has the same components as the wavelength-division multiplexing transmission equipment 141, by way of optical fibers 131 and 132. A transponder 113 is interposed between an optical transceiver 110 and the wavelength-division multiplexing transmission equipment 141. The transponder 113 comprises a transmission transponder 114 that converts a signal received from an optical transmitter 111 into a signal to be subjected to wavelength-division multiplexing, and a reception transponder 115 that converts a signal received from the wavelength-division multiplexing transmission equipment 141 to a signal that can be received by the optical transceiver.
An optical signal sent from the optical transmitter 111 included in the optical transceiver 110 is transferred to the transmission transponder 114 included in the transponder 113, and converted into a signal that has any of wavelengths managed in the form of a grid (managed in the order of nanometers) and that is supported by the wavelength-division multiplexing transmission equipment. The optical signal having the wavelength thereof converted falls on the multiplexer 142 included in the wavelength-division multiplexing transmission equipment 141. The optical signal is then combined with other optical signal, whereby a wavelength-multiplexed signal is produced. The wavelength-multiplexed signal propagates along the optical fiber 131, and then reaches a demultiplexer 153 included in the wavelength-division multiplexing transmission equipment 151. The wavelength-multiplexed signal is then separated into signals of different wavelengths. The separated optical signals are transferred to the reception transponder 125 included in the transponder 123, converted into signals supported by an optical transceiver, and then received by the optical receiver 122.
Even on the opposite side of the system, an optical signal sent from the optical transmitter 121 is received by the optical receiver 112 via the transponder 124, multiplexer 152, optical fiber 132, demultiplexer 143, and transponder 115. Thus, when a transponder in which wavelengths are managed for the purpose of wavelength-division multiplexing is interposed between an optical transceiver in which wavelengths are not managed, such as, an Ethernet transceiver and wavelength-division multiplexing transmission equipment, transmission of a large throughput (Ethernet-based wavelength-division multiplexing transmission) can be realized inexpensively.
Moreover, some transponders have a loading facility for performing encoding that is intended for error detection or error correction, signal addition that is adapted to a control signal to be transferred between transponders, or reshaping or reproduction of a wave. When this kind of transponder is employed, a certain delay time is produced between a received signal and a transmitted signal.
In the system having the configuration shown in FIG. 6, when the optical transceiver 110 and optical transceiver 120 perform fault notification according to different state transition diagrams, that is, when the optical transceiver 120 performs fault notification according to the state transition diagram of FIG. 3 and the optical transceiver 110 performs fault notification according to the state transition diagram of FIG. 5, malfunction may occur at the time of starting up the optical transceiver 110. This phenomenon will be described below.
FIG. 7 shows a recovery sequence to be followed when one of the optical transceivers that are included in the configuration shown in FIG. 6 and connected opposite to each other, that is, the optical transceiver 110 is rebooted (restarted). FIG. 7 also shows the state of the transmission transponder 114 connected to the optical transmitter 111. For brevity's sake, the description of the actions of the reception transponder 125, opposite transmission transponder 124, and opposite reception transponder 115 will be omitted.
When the optical transceiver 110 is rebooted, the opposite optical transceiver 120 detects a fault, changes states from state 0 to state 1, and transmits a fault detection signal to the optical transceiver 110. When the rebooting of the optical transceiver 110 is completed, the fault detection signal transferred from the opposite transmitter is received. The optical transceiver 110 changes the state thereof into state 2, and transmits a standby signal to the optical transceiver 120.
Assume that a delay occurs in the transmission transponder 114 after reception of the standby signal until transmission thereof. If a delay in transmission of the standby signal occurs in the transmission transponder 114, the optical receiver 120 delays by the delay time in detecting the standby signal and returning to state 0. Consequently, the optical transceiver 110 delays in receiving a data signal. At this time, before the data signal reaches the optical transceiver 110, if a certain period of time T1 described in conjunction with the state transition diagram of FIG. 5 elapses after the optical transceiver 110 enters state 2, the optical transceiver 110 changes the state thereof into state 3. Consequently, transmission of the standby signal is suspended and recovery work itself is suspended. The optical transceiver 110 suspends transmission during a certain period of time T2. Thereafter, the optical transceiver 110 returns to state 2 and transmits the standby signal. However, since the delay has occurred in the transmission transponder 14, if the optical transceiver 110 cannot receive the data signal during the period of time T1, the optical transceiver 110 reenters state 3. Transmission of the standby signal is suspended. The optical transceiver 110 repeats the same actions and falls into a loop state in which state 2 and state 3 are repeatedly alternated. Eventually, it becomes impossible to recover the optical transceiver 110 after rebooting.
A delay occurring in a transponder is attributable partly to a startup time required by the transponder. Although no signal input has been detected in the transponder so far, if production of a signal input is initiated, the internal circuit of the transponder must be started in order to provide a signal output. This causes a delay. Moreover, when the transponder is recovered from the no-signal state, if human manipulations are required, a delay time is naturally produced until a worker autonomously performs recovery work. If a slow-start facility that does not abruptly transmit a large-power signal but increases power little by little is included, a delay occurs for a period of time required until the power is increased to the level permitting a receiver to recognize the signal.
Conventionally, optical transceivers, transponders, pieces of wavelength-division multiplexing transmission equipment, and opposed stations included in a WDM system are manufactured by the same manufacturer. However, as far as Ethernet-based wavelength-division multiplexing transmission is concerned, if the transponders and pieces of wavelength-division multiplexing transmission equipment are manufactured by the same manufacturer, the wavelength-division multiplexing transmission equipment manufacturer is requested to provide a product to which diverse optical transceivers manufactured by numerous manufacturers can be connected.
As described in the Japanese Unexamined Patent Application Publication No. 2001-217778, whichever of a method employing as a standby signal a signal (short pulse train) whose duty factor is small and a method employing a signal whose level or power itself is low is adopted, unless transponders support the method, recovery from a fault is impossible. FIG. 8 shows a sequence to be followed when an optical transceiver is rebooted. When the optical transceiver 110 is rebooted, the optical transceiver 120 opposite to optical transceiver 110 detects a fault, changes states from state 0 to state 1, and transmits a fault detection signal to the optical transceiver 110. When the rebooting of the optical transceiver 110 is completed, the optical transceiver 110 receives the fault detection signal transferred from the opposite transmitter, enters state 2, and transmits a standby signal to the optical transceiver 120. Although the transmission transponder 114 receives the standby signal, if the transmission transponder 114 does not transmit the standby signal but intercepts transfer of the standby signal, a data signal is not returned to the optical transceiver 110. Even in this case, after a period of time T1 elapses, the optical transceiver 110 changes the state thereof into state 3, and suspends transmission of the standby signal. After a period of time T2 elapses, the optical transceiver returns to state 2 and resumes transmission of the standby signal. If the transmission transponder 114 intercepts transfer of the standby signal, the optical transceiver 110 reenters state 3. Likewise, the optical transceiver 110 falls into a loop state in which state 3 and state 2 are repeatedly alternated, and is not recovered from a fault.
The phenomenon that the transmission transponder 113 intercepts transfer of a standby signal takes place in a case where although a signal (short pulse train) whose duty factor is small or a signal whose level or power itself is low is adopted as the standby signal to be transmitted from the optical transceiver 110, the transmission transponder 114 does not support the special standby signal and does not therefore recognize a received signal as an effective signal. In particular, if a special signal unique to a manufacturer of an optical transceiver is adopted as the standby signal, the transponder cannot deal with the signal.
In order to solve the foregoing problem attributable to the interaction between the fault notification facility included in a transceiver and signal processing performed in a transponder, the fault notification facility of the transceiver must be improved and the signal delay occurring in the transponder must be overcome. Otherwise, the problem is solved by temporarily invalidating the fault notification facility itself. However, if the optical transceiver has already been incorporated in a router or optical transmission equipment, upgrading of the optical transceiver or modification of settings is often hard to do. Moreover, in an equipment installation site or the like, there is difficulty in modifying settings for lack of a satisfactory equipment setting environment or equipment setting data. These cases cannot be coped with by updating the optical transceiver or transponder.